A previous question that I asked got me thinking about simultaneity, and I have a couple of questions on the subject. Concerning the Einstein's train thought experiment. The observer on the train perceives the lightning to strike the front of the train first. So in his reference frame he does not consider the two lightning strikes to be simultaneous, as the observer on the platform does. But let's say that there was a clock at the front of the train and one at the back of the train. Both clocks were knocked out by the lightning strikes. If the observer goes to restart the clocks he will realize that both clocks stopped at exactly the same time. He will also realize that the strikes were therefore simultaneous after all. Doesn't this mean that even in his reference frame the lightning strikes occured simultaneously, whether he perceived them as happening that way or not? If he's a bright guy he also realizes that this means that he must be in motion. If we do the same thing for the observer on the platform, then he too can check the clocks to verify his perception that the strikes occured simultaneously. What this leads me to wonder is.....is it possible to create a sort of temporal GPS? Could an advanced enough civilization set up a system of beacons to act as an absolute time keeper. Against which anyone can determine their position and motion, by comparing their shipboard "clock" against these beacons? I also wonder if there are any objects in the universe that could act as natural beacons, against which a space traveler could compare his perceived rate of motion? I don't know, but perhaps you could somehow employ pulsars. Just wondering.